The present invention relates to solar energy collectors or cells comprising a fixed spherical mirror whose centre is located above the mirror and a boiler which is movable around the centre of the mirror in which circulates a heat-transfer fluid.
One of the objects of the invention is to construct solar collectors having a power of the order of 1 MW which serve to heat a heat-transfer fluid to a temperature of approximately 350.degree. C. in order to provide the steam required for supplying a thermal power station.
Solar energy collectors are already known which have a fixed spherical mirror which rests on the ground via its convex face having a substantially vertical axis at low latitudes or a slightly inclined axis at higher latitudes.
One of the objects of the present invention is to improve this type of collector, more particularly with reference to the boiler so as to be able to collect energy, particularly in the high energy concentration zone adjacent to the paraxial focus.
It is pointed out that the paraxial focus of a spherical mirror is located at a distance between the centre and the surface of the mirror on the axis passing through the centre of the mirror and through the centre of the sun.
Another object of the present invention is to improve the mirror so as to avoid a loss of efficiency when the height of the sun varies, without projecting a shadow onto the mirror and without making it necessary to increase the angular aperture of the mirror.
A collector according to the invention comprises in per se known manner a fixed spherical mirror whose centre is located above the mirror, a radius of curvature R and a paraxial focus; a movable boiler in which circulates a heat-transfer fluid, said boiler having an axis which passes through the centre of the mirror; and means for pivoting the boiler axis around the centre of the mirror, whilst maintaining it directed towards the sun.
The objects of the invention are achieved by means of a collector of the type indicated hereinbefore which has a boiler in the form of two coaxial boilers which rotate about their common axis, namely an elongated low concentration boiler with two axially opposed ends, a lower end located in the vicinity of the mirror surface via which a heat transfer fluid enters and an upper end; and a high concentration boiler which is located in the vicinity of said paraxial focus and which is separated from said upper end of the low concentration boiler by an intermediate space.
The high concentration boiler preferably has the form of a cavity which rotates about the axis common to the two boilers having two axially opposed ends, a lower open end and an upper closed end, said high concentration boiler having a coil in which circulates a heat-transfer fluid, said coil covering the walls of said cavity and has an intake port for the fluid located on the periphery of said lower end and a discharge port for the fluid located in the centre of said upper end.
Preferably the diameter of the lower end of the high concentration boiler is between 0.05 and 0.06 times the radius of curvature R of the mirror and this end is located at a distance from the centre of the mirror which is between 0.54 and 0.57 R.
The low concentration boiler comprises either a coil wound round a coaxial cylinder with the high concentration boiler, or exchanger tubes arranged parallel to the generating lines of a cylinder and to the boiler axis.
A mirror according to the invention comprises in per se known manner a spherical reflecting cap, whose convex portion is placed on the ground.
The second object of the invention is achieved by means of a movable visor constituted by a reflecting spherical sector which extends said cap over a portion of the periphery and by means for rotating said visor around the cap axis in such a way that the boiler is constantly in the plane of symmetry of the visor.
Preferably in a plane perpendicular to the axis of the cap the visor has an angular aperture between 90.degree. and 120.degree. and in a meridian plane passing through said axis an angular aperture between 10.degree. and 25.degree..
The result of the invention is a novel product constituting a solar energy collector or cell which makes it possible to heat a heat-transfer fluid to a temperature between 100.degree. and 500.degree. C. and consequently produce the steam making it possible to drive a turbine coupled to an a.c. generator.
For example a collector according to the invention having a mirror whose radius of curvature is of the order of 25 m and a semiapertural angle of 60.degree. has a reflecting surface of 3,000 m.sup.2 and makes it possible to collect a thermal power on the boiler of the order of several megawatts.
An advantage of the collectors according to the invention is that they permit a relatively high energy concentration onto boilers with a limited surface area, whilst obtaining high temperatures with a high efficiency. The special construction of the present boiler with a first low concentration boiler followed by a second high concentration boiler which is separated from the former makes it possible to pre-heat the heat-transfer fluid to a temperature of approximately 250.degree. C., prior to optionally passing it into the high concentration boiler in order to raise the final temperature to approximately 350.degree. C., or alternatively it can be used separately.
The rotary visor makes it possible to increase the quantity of energy reflected onto the boiler when the height of the sun decreases without there being any danger of creating a shadow which is directed onto the mirror. Thus, it is possible, even outside equatorial zones to construct mirrors with a vertical axis which can be more easily placed on the ground, whilst retaining a good useful reflecting surface.